Bonding to Existing Concrete

posted by Bob

Fact: Fresh wet concrete does not normally bond well to existing dry concrete. Do you remember elementary school where one of the subjects on which you were graded was “plays well with others”? Concrete would have gotten an F. There is nothing in basic portland cement that will act as a bonding agent. Portland cement concrete works well in mass and provides great compressive strength but not bond.

Concrete is marvelous stuff but in time it will deteriorate. When it does, you either have to patch it or replace it. Assuming that it is structurally sound the least expensive alternative is to patch it. However patching it requires some attention to detail or your patch will not last. So that you don’t waste too much time or money, we should probably discuss what “structurally sound” means. If your sidewalk has either heaved or dropped at almost every joint, repairing it will not provide a long-term solution. The slabs are likely still moving. If your slab has so much sand and gravel on the surface that despite sweeping and sweeping and squirting and squirting it just keeps coming back, don’t waste your time on repairs. If you have multiple cracks that run so deep that they appear to run through the slab, a repair would only be temporary. The solution to all of these problems involves a jackhammer and bags of one of the Sakrete concretes.

Since this discussion is on the best way to bond concrete, we will assume that your slab is good.

There are a variety of Sakrete concrete repair products available to fix concrete that has begun to deteriorate. However, without good surface preparation, none of them are going to perform satisfactorily. All loose sand, gravel, dirt, leaves etc. must be removed. This can typically be done with a garden hose and a good nozzle. Tough areas may require a pressure washer or mechanical abrasion. The two toughest areas to cover are those with oil and tree sap. Both of these will work their way down into the concrete. Simply washing the surface isn’t sufficient. If the stains do not run too deep, you can chip away the concrete using a hammer and chisel. Don’t forget the goggles (not just glasses) as this process will throw concrete all over the place. Also, keep your thumb out of the way. If the spots are too large or too deep for this to be practical, you may need a sealer to cover the stains before patching.

There are two basic methods for bonding a portland cement based product to existing concrete; 1) chemically and 2) mechanically.

Let’s discuss the mechanical approach first since it is really used in both approaches. The most effective way to ensure a really good bond is with a scratch coat. This is simply a very wet coat made up by mixing the repair product with water. Mix up a small amount of the repair material to a soupy consistency. You don’t need to measure the water-just turn the stuff into slop. Then, using a gloved hand or a rag, smear the material onto the area to be patched. Just think finger painting from kindergarten. The technique is about the same. Apply pressure to ensure that as much as possible is shoved into the nocks and crannies. You only need a thin coat. It is not necessary for this scratch coat to dry. By the time you get the repair material mixed it will be ready. Then mix up additional repair material to the proper consistency and apply over this thin scratch coat.

The chemical approach involved mixing up a liquid bonding agent that helps bond new concrete products to old. Products like Sakrete Top'n Bond and Sakrete Flo-Coat already contain polymers that greatly improve the bond of portland cement and should NEVER be used with a liquid bonding agent. I know in America bigger is better but it’s just not so with these products. Other products like Sakrete Sand Mix and Sakrete Fast Set Cement Patcher benefit from the use of a liquid chemical bonding agent such as Sakrete Bonder/Fortifier. When using a liquid bonding agent, paint the bonder onto the existing concrete and allow it to dry until it is tacky. This usually takes only a few minutes. Then apply the repair material. Just as in the process described above, after the bonder has become tacky apply a scratch coat and then apply the repair material. The most effective way to ensure that the bonding agent gets into the existing concrete is to apply it directly using a brush or rag. It can be sprayed if you happen to have a sprayer. Although the directions say that you can use it as part of the mix water, direct application works better.

If you are doing a large area and a scratch coat isn’t practical you will need to spray the surface with water before you apply the repair material. On a warm day, the existing concrete surface will be hot enough to suck the water out of the repair material. In addition, some concretes are quite porous and will rob water from your repair material. If too much water is lost into the old concrete there will not be enough water to hydrate all of the cement particles and a lower strength material will be the result. Concrete simply will not bond to all substances. Paint, oil, glue from old flooring tiles are just a few. You must mechanically remove these materials if you want the job to last.

Once the job is complete, you can do a quick check to see if the bond was successful. Wait at least 24 hours and then tap “gently” on the patch using a hammer or some other dull object and listen for a hollow echoing sound. If you just get a dull thud then the material has bonded well. If you get a hollow sound, the material has not bonded and will crack in time. Which means it is back to the beginning of today’s topic. Here is hoping your concrete work comes across as a dull thud (not like some of my party guests) rather than a hollow endeavor.

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701 USER COMMENTS

Rick, the biggest issue is going to be the oil stains. You will need to try to get the oil up without using solvents. A solvent based cleaner will only break down the oil and cause it to leech further into the concrete. Use oil dry materials to pull it out of the concrete for better results and then clean with a detergent based concrete cleaner. If the stains still bead up and if the oil is too deep then you will have to grind the surface until the oil has been removed. Once the spills are taken care of then it will depend on the depth of the repair that you are left with to determine which product to use. The Flo-Coat and the Top'n Bond both can be applied from 1/2" down to a featheredge. The Fast Setting Cement Patcher can be applied from 2" down to a 1/4". You may need to use multiple products depending on the severity of the repair. Fix the larger depressions first with the Fast Setting Cement Patcher and in about an hour, resurface the area with the Flo-Coat Concrete Resurfacer. The surface must be clean and free of any loose material, dust, debris, paints, sealers, and oils. The material also needs to be applied while the surface is damp.
- Lee-Technical Service
Monday, September 26, 2016 at 12:36 PM

I have a 40+ year old suspended slab in my garage. The concrete is spalled, uneven and stained with oil and other stuff from all of the years of abuse. I need to resurface it as it collects water due to a door seal that won't seal due to the unevenness of the surface. Maximum thickness of the topping would be 2 inches but less would be better. What can I do? Replacement would require demolishing the entire structure as the structure sits on top of the slab.
- Rick Morgan
Friday, September 23, 2016 at 3:46 PM

Ron, no you cannot use the Top N Bond to resurface a swimming pool. You will have to use a compatible product that will work with the existing pool surface. You could check with a local pool supply store.
- Lee-Technical Service
Thursday, September 22, 2016 at 3:10 PM

Can Sakrete Top'n Bond be used to do resurface on a concrete pool ..pool concrete was mixed with sahara waterproofing
- RON
Thursday, September 22, 2016 at 1:31 AM

Roger, the new pour will have to be pinned to the existing foundation of the garage. To get this information you will have to contact your local Building Code Department for the proper procedure and installation so that the addition will be code compliant.
- Lee-Technical Service
Tuesday, September 20, 2016 at 9:21 AM

I want to make concrete garage base wider bye 1ft the base is16ft long will it be OK just to pour concrete in ,or if not how do you make sure it is joined I'm having a wider garage
- Roger
Tuesday, September 20, 2016 at 3:43 AM

Danel, unfortunately anything that you do to repair this will only be a Band-Aid. This would be considered a cold joint and would not hold up for long. If you have the original pieces you could try to epoxy them back in place. If you have to pour concrete in the new area, then you will need to drill and epoxy in some anchor hooks to help hold the new pour in place. After the hooks are set, brush Bonder and Fortifier onto the surface that your new concrete will be touching. Mix the concrete and pour it into your area. Wait 24 hours before you pull off the framing. But keep in mind that this will always be a cold joint and that it will not hold up for too long if the area has any movement or stress placed on it.
- Lee-Technical Service
Thursday, September 15, 2016 at 1:14 PM

Matt, sealing the surface would not be a good idea if you intend on laying tile later. The sealer would have to be removed mechanically. I would recommend doing the repairs and wait to see if you are going to like it. If at that time you want to leave it, then seal the surface. If not the area will only have to be cleaned to tile is desired.
- Lee-Technical Service
Thursday, September 15, 2016 at 11:18 AM

The edges on my porch (concrete slab) have broken off due to improper installation of the railings. I am removing the railings and would like to add 5 inches of concrete to reform them Which product would be best? The new concrete would have to bond to the existing concrete slab. This is a diy project.
- Danel
Thursday, September 15, 2016 at 8:43 AM

1. I recently had vinyl tiling & mastic removed from a concrete slab (at grade, not basement). The mastic was removed mechanically (grinding). 2. I plan to patch small damaged areas at the edges and 1-2 cracks, seal it with an appropriate product, and live with it for a year or so. 3. In a year or two, if we decide we don;t like the look, I would like to finish the floor with porcelain tiles. Is there a sealer that I can apply at Step 2 that would *not require removal* at Step 3? Is is a good idea -- or even possible -- to apply thinset and tile to concrete floor that has been sealed, without chemical or mechanical removal of the sealer? Thanks for advice!
- Matt N
Wednesday, September 14, 2016 at 1:32 PM

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